i.] Structure of the Bacterium-cell. 3 



defined by natural history and not by physiological characters 

 only, Bacteria are as little related in structure and development 

 as bats are to birds ; the relationship is even less, because there 

 are a few, though only a few, true Bacteria which contain chlo- 

 rophyll and decompose carbon dioxide, and which are therefore 

 not Fungi in the physiological sense. 



For these reasons we shall be more strictly correct if we 

 speak on the present occasion of Bacteria rather than of Fission- 

 fungi ; but so long as we are quite clear as to the difference in 

 the meaning of the two words, it is a matter of no importance 

 which we use. 



The conformation, structure, and growth of Bacteria are 

 extremely simple, if we put out of sight certain phenomena of 

 propagation and consider only the vegetative state. 



Bacteria appear in the form of round or cylindrical rod-shaped, 

 rarely fusiform, cells of very minute size. The diameter of the 

 round cells or the transverse section of the cylindrical cells is in 

 most cases about o'ooi mm. ( = i micromillimetre = i /z) or even 

 less. The length of the cylindrical cells is 2-4 times the trans- 

 verse section, rarely more. There are only a few forms with dis- 

 tinctly larger dimensions. Putting aside, for later consideration, 

 the forms from the group of Beggiatoa, Crenothrix and their 

 allies, which differ to some extent in this and other respects from 

 the rest of the Bacteria, the greatest breadth yet observed is 4 /a, 

 the measurement given by Van Tieghem for the rod-shaped 

 cells of Bacillus crassus. 



We are obliged to apply the term cells to those minute bodies, 

 because they grow and divide like plant-cells, and also because 

 all that we know of their structure agrees with the corresponding 

 phenomena in plant-cells. It is true that their small size does 

 not permit of our going at present very deeply into the minutiae 

 of their structure. Cell-nuclei, for instance, have not yet been 

 observed in them ; but this is the case in many small cells of 

 other plants of a low order of growth, especially Fungi, and till 

 recent times it was the case with respect to all fungal cells. 



B 2 



